What was the average decision speed pf Alpha Zero in the recent Stockfish match?
No, ALL computer chess experts were surprised about the outcomes of the match. If you require references, please start a new question.
Your question is simple...

https://arxiv.org/pdf/1712.01815.pdf
... We evaluated the fully trained instances of AlphaZero against Stockfish, Elmo and the previous
version of AlphaGo Zero (trained for 3 days) in chess, shogi and Go respectively, playing
100 game matches at tournament time controls of one minute per move ...

One minute per move. Stockfish would use the one minute if there is more than a single legal move, otherwise it'd move immediately. So the average time for a move for Stockfish is about 57s to 60s.
There is no source code for AlphaZero. However, it's hard to believe the system wouldn't take advantage of the minute it was given. The expected time for AlphaZero should be also 57s to 60s.